The present invention generally pertains to digital signal processing and is particularly directed to a system for recognizing a received encoded digital signal as being from a particular transmitter.
The recognition of a received encoded signal as being from a particular transmitter is important in systems wherein an apparatus at one location is remotely controlled from another location. In a garage door actuation system, for example, an encoded signal transmitted from an automobile actuates a garage door opener only if it is recognized upon receipt as being from a particular transmitter.
One technique of encoding the transmitted signal so as to define a discrete communications channel between only a selected receiver and a particular transmitter has been to allocate a different carrier frequency to each such channel. However, the number of available discrete carrier frequencies is limited and systems using carrier frequency allocation often have been actuated inadvertently or accidently by stray signals having the same carrier frequency.
In an improved transmitter and receiver encoding system described in U.S. Pat. No. 3,906,348 to Willmott, a plurality of two-position switches establish a binary serially transmitted code at the transmitter that will energize only the particular receiver which is set to the same binary code by an equivalent number of two-position switches at the receiver. Although there are a limited number of carrier frequencies that can be used in transmitters and receivers of this type, the total number of available combinations is many times that available wherein only different carrier frequencies are used to isolate the different transmitter and receiver combinations. Willmott's system also provides that in the unlikely event that a similar transmitter and receiver are located within radio frequency range in which the same radio frequency channel and the particular binary coding has been selected, that the binary coding can be changed in the transmitter-receiver to a different coding by merely resetting the two-position switches and thus eliminating any possible interference.
In Willmott's system a digital logic signal derived from the received encoded signal is compared with a programmed digital logic signal that is encoded by the settings of the two-position switches at the receiver; and the received signal is recognized as being from the particular transmitter upon the receipt of four successive "valid" encoded signals.